Pumps which are adapted for the transport of liquids and slurries containing solid matter may be equipped with means arranged on the suction side of the pump for cutting solid matter which is entrained in the liquid into smaller fractions that are sized to pass through the pump. These pumps are often referred to as chopping pumps, many of which are structured as centrifugal pumps providing an axial intake flow of liquid, whereas the discharge flow is radial as seen with respect to a pump wheel.
Chopping pumps are known from the literature. For example, EP 0,395,604 A1 and U.S. Pat. No. 4,108,386 both disclose pumps having cutting impellers mounted in coaxial relation with a pump wheel and co-rotating therewith. The shearing action is provided from cutting edges arranged at a cylindrical/axial interface between the rotating impeller and a stationary ring-shaped insert which surrounds the impeller at the pump intake. In such case, the axial relation between co-operating cutting edges of the insert ring and cutting impeller is not critical to the shearing action, but rather the radial relation between these components.
WO 2006/058605 A1 discloses a chopping pump wherein shearing action is provided at a radial interface between an axial impeller, which is mounted in coaxial and co-rotational relation to a pump wheel, and the downstream side of a perforated cutting plate covering the pump intake. In such case, the shearing capacity is crucially depending on an accurate axial clearance between interacting cutting edges on the upstream end face of the impeller and on the downstream face of the intake cutting plate, respectively. To this purpose, spacer sheets need to be interposed between the intake cutting plate and the pump housing for adjustment of the axial clearance. Obviously, the adjustment is made in a final mounting step as the impeller is threaded onto the drive shaft whereby the impeller also locks the pump wheel, which has previously been keyed onto the drive shaft, in its axial position. Thus, setting of an effective shearing interaction between the axial impeller and the intake cutting plate involves the axial relation between all components, including the pump wheel.
A state of the art chopping pump is illustrated in FIG. 1 of the drawings. The chopping pump of FIG. 1 will be briefly discussed below, focusing mainly on the components that are of importance for the shearing operation.
The prior art chopping pump of FIG. 1 comprises an impeller pump wheel 1 which is journalled for rotation in a pump housing 2. The pump housing 2 has an axial intake 3 on the suction side and a radial discharge 4 on the pressure side for liquid transport effectuated by the pump wheel in rotation. Arranged co-axially with the pump wheel, and co-rotating therewith, is a cutting wheel 5. In operation, the cutting wheel rotates on the upstream side of a perforated cutting plate 6 which is stationary with respect to the pump housing. More exactly, the cutting plate 6 is bolted in covering relation with a central opening 7 that is formed through a suction plate 8, which is bolted to the pump housing at 9. Radial cutting edges 10, formed on the downstream side of the cutting wheel, co-operate in shearing interaction with edges of perforations 11 that are formed through the cutting plate. Any solid matter of some length that is sucked in through the perforations 11 is cut by the cutting wheel in relative rotation to the cutting plate.
The rotating components, i.e. the pump wheel 1 and cutting wheel 5, are carried in the end region of a drive shaft 12 which is journalled in the pump housing and is driven by a motor for rotation. The shaft 12 reaches through central bores that are formed in the pump and cutting wheels respectively. The pump wheel and cutting wheel are both non-rotationally keyed to the shaft 12 through splined connections, and are fixed axially to the shaft end by means of a locking bolt 13 which is threaded into a blind bore mouthing in the shaft's end. Evidently, setting of a proper axial clearance between the cutting wheel and the cutting plate involves the axial relation between all components, including the pump wheel.
The axial clearance between cutting wheel 5 and cutting plate 6 is established and adjusted in connection with the mounting procedure, which will now be described.
In a first mounting step the pump and cutting wheels are inserted on the shaft end, and secured axially through the locking bolt 13 while adjusting an axial clearance between the pump wheel and pump housing by means of spacer washers that are previously installed on the drive shaft 12. In a next step the suction plate 8 is bolted to the housing and adjusted with respect to a clearing distance between the pump wheel and the suction plate. Then the cutting wheel is removed by loosening the locking bolt 13, the pump wheel now able to rest on the downstream face of the suction plate. With the cutting wheel removed, the cutting plate 6 can be bolted to the suction plate 8 whereupon the cutting wheel is again installed on the shaft's end and the locking bolt is applied to re-establish the axial position of the pump and cutting wheels on the drive shaft 12. Additional spacer washers may then be necessary to install on the drive shaft, between the cutting wheel and the pump wheel, in order to provide a clearance and a degree of adjustment. The final setting of a minimum clearance between the cutting plate and the cutting wheel is performed by adjustment of the axial position of the suction plate 8, using the bolts 9 or separate set screws.
Obviously the mounting and adjustment procedure is time-wasting, and the method relying on an operator's skill to ensure a reproducible clearance at all times. But since the ability to cut down solid matter that would otherwise block the liquid intake is crucial to the chopping pump's operation, the accurate axial clearance has always to be ensured. It is thus a technical problem to improve the prior art chopping pump such that an operative axial clearance between cutting elements is always reproduced upon mounting, and by which the risk of non-proper mounting is eliminated.